1. Field of the Invention
The present invention relates generally to wastewater treatment systems and methods, and, more particularly, to such systems and methods using bioremediation techniques.
2. Related Art
Engineered wetlands for wastewater treatment have been under development since the work of Kathe Seidel in the 1950s. Three basic hydraulic configurations in treatment wetlands are known: surface flow (SF), subsurface horizontal flow (SSHF), and vertical flow (VF). Of these, SF and SSHF wetlands are believed the most common.1 Even though an early wastewater treatment wetland design by Seidel utilized vertical flow,2 design criteria are still considered experimental for vertical flow wetlands. Design criteria for VF wetlands are absent from recently published design texts that have extensive expositions of surface flow and horizontal subsurface flow wetland designs.
In the wetland treatment design literature, surface flow wetlands are typically presented as having two significant design shortcomings. The first shortcoming is that surface flow wetlands cannot typically achieve the low BOD5, ammonia, and total suspended solids (TSS) effluent concentrations required for advanced tertiary treatment. The other shortcoming is that surface flow treatment wetlands can serve as sites for breeding disease vectors and nuisance pests. Therefore, surface flow wetlands are typically restricted to applications not requiring advanced tertiary treatment by or sites where vector control is not a critical concern.
Subsurface horizontal flow wetlands tend to provide better BOD5 and TSS treatment than SF wetlands. Despite this advantage, both BOD5 and TSS effluent values from SSHF wetland cannot reliably be expected to meet tertiary treatment standards. Nitrification in SSHF wetlands is typically poor. An advantage of SSHF wetland design is that with no exposed water surface there is no place for disease vectors to breed, but in practice this advantage is often not realized because of surfacing and ponding of wastewater resulting from clogging of wetland media.1 
Surfacing and ponding of wastewater in SSHF wetlands is believed to be an inherent design weakness. Interstices in gravel media eventually fill with organic and inorganic substances carried in or generated from the wetland influent.3 Channeling then occurs within the wetland media, degrading treatment. Horizontal flow path velocities are insufficient to carry inorganic fines and recalcitrant organic materials through the media to the wetland outlet. Media in the wetland inlet zone then clogs, forcing wastewater to the surface. Although wastewater will eventually submerge again into the media downstream, some ponding is unavoidable in this situation, and disease vectors can then breed in the ponded wastewater.
The role of plants in SF and SSHF wetlands is substantially different. In SF wetlands plants are rooted in a soil substrate. The stalks and leaves of plants contact the wastewater, not the roots. Stalks and leaves accumulate into a thick thatch cover. It is believed that the plant thatch surface area contributes significantly to the treatment of wastewater. In SSHF wetlands water flows through the gravel media in which the plants are rooted. Theoretically, the contact of wastewater with plant roots plays a significant role in treatment.
There is growing evidence that plants do not play a significant treatment role in SSHF wetlands. Results from studies comparing vegetated and unvegetated subsurface flow wetland treatment systems indicate that plants do not significantly impact treatment,1,4 even though there is strong evidence that the presence of roots in SSHF wetlands significantly affects the composition of microbial populations.5 Findings of little or no contribution to treatment from plant roots in SSHF wetlands appear to arise from the relationship between roots and media, and the growth characteristics of roots. The treatment effect of roots is likely to be poorly distinguished from that of media if the media surface area is very large compared with that of plant roots. Moreover, one of the inventors (D. A.) has observed that, in horizontal subsurface flow wetlands, roots tend to grow little below the permanently wetted media surface, creating only a shallow zone of root penetration. The greater hydraulic resistance created by the plant roots reduces wastewater flow in this zone. A dead zone frequently results, owing to the deposition of organic material and the lack of circulation and re-aeration in the root zone.1 
The potential treatment role of roots cannot be determined if there is minimal root contact with wastewater. Surface-loaded, vertical-flow wetlands4 theoretically avoid the root-zone flow problems of horizontal subsurface flow wetlands because surface loading forces flow through the root zone. Distinguishing between the treatment effects of media and roots may still be difficult, however. Root zone architecture may also be a confounding factor because of plant species variations and changes occurring during plant maturation. Species-specific root architecture may affect treatment directly7 and indirectly by altering media porosity1,8 and thus altering wetland hydraulics.
The basic hydraulic flow path for VF wetlands is for wastewater to be introduced at the wetland surface, pass through media and plant roots, then to flow out of the wetland via an underdrain system. Designs vary considerably in how wastewater is distributed on the wetland surface, in media composition and configuration, duration of flooding if used, depth of flooding, and recycle if used.
Most work with VF wetlands has been done in Europe, where European vertical flow wetland designs commonly employ fine, sharp sand at the surface, underlaid with a coarser media.9 Plants root in the fine sand, and the low hydraulic conductivity of the fine sand forces a temporary free water surface. Slow percolation through the sand layer is thought to aid treatment. After completely draining, the previously flooded VF wetland cell is allowed to rest for a period, usually a few days, to permit reaeration of the sand layer. Without reaeration the sand in the interstices would eventually clog with accumulated wastewater constituents and biomass growing on wastewater nutrients.10 
The European VF wetland design began with the work of Kathe Seidel11; however, the design development advances are still relatively new because interest in vertical flow wetland design for wastewater treatment only began to capture the interest of wetland designers in the 1990s.11 A similar design for dewatering and composting biosolids in sand filters planted with Phragmites reeds has been in use since the 1960s.12 
European VF wetland designs appear to provide superior BOD5 removal, nitrification, and total nitrogen removal than SF and SSHF wetlands, but removal of TSS may be better in SSHF wetlands.11 Some treatment wetlands are designed in combination, employing a VF wetland for nutrient removal, then followed by a SSHF wetland for TSS removal.11 Vertically loaded wetlands in series have been investigated as well, indicating treatment advantages to this approach.4,13 
Vertical flow wetlands are often designed to have a period of filling followed by a period of draining. When filled by wastewater, bacterial metabolism within the media depletes dissolved oxygen, producing anoxic or anaerobic conditions. As water drains, air is drawn down into wetland media.13 Draining is important to permit aeration of wetland media. Fill and drain vertical fluctuation of water levels in vertical flow wetlands is therefore periodic. Drain and fill cycles with a period of approximately a day or less are termed tidal flow.13 Previously known tidal flow systems are characterized by poor denitrification performance, with the exception of a reciprocating tidal flow system as taught by Behrends.
In the United States, designs have been produced on European models.14 The Tennessee Valley Authority has disclosed a reciprocating wetland design (U.S. Pat. No. 5,863,433) that appears to achieve advanced tertiary treatment. In this system forward flow passes through series of paired VF wetland cells, with wastewater pumped back and forth between the paired cells several times a day. Such a reciprocating action confers treatment advantages. Each drop of wastewater on average is subjected to multiple passes through wetland media and plants in the reciprocating flow. Because the pumping action drains each cell several times per day, wetland biofilms reaerate with the same frequency. As the water level drops, atmospheric oxygen is pulled into the media bed in a process of hydrodynamic air pumping. Reaeration of biofilms in the wetland media occurs very rapidly, in about 30 seconds on exposure to the atmosphere.
Other vertical flow wetland designs include an upflow VF wetland design in Australia (Australian Pat. No. 461902), which not appear to consistently achieve advance tertiary effluent quality with wastewater effluent. Other researchers have experimented with single-pass downflow VF wetland wastewater treatment systems,15 and combined downflow and upflow VF wetland systems for surface water remediation.16 
Lagoon wastewater treatment systems comprise large basins in which wastewater is retained for many days or weeks. Depending upon organic mass loading and design, lagoons may be anaerobic, aerobic, or facultative. A facultative lagoon has an upper layer that is aerobic and a lower layer that is anaerobic. Typically cyanobacteria or algae dominate such lagoons. Aerobic and anaerobic cycling may be diurnal in nature, depending upon photosynthesis and wind-induced mixing.
The advantage of lagoons is their low capital and operating costs. However, lagoons demand large land footprints, owing to the long residence times; in addition, they are not capable of achieving advanced treatment, typically reaching secondary treatment standards at best. Algal growth in lagoons often creates effluent TSS concentrations that are much higher in BOD and TSS than secondary treatment standards would permit, and the filtration of algae from lagoon effluent is difficult.